1. Field of the Invention
The present invention relates to an organic light emitting element having an anode, a cathode, and a layer that contains an organic compound capable of emitting light upon application of electric field (the layer hereinafter referred to as organic compound layer), and to a light emitting device using the element. Specifically, the present invention relates to an organic light emitting element that has a wider choice of materials than before while keeping the same level of light emission as prior art. In this specification, the term light emitting device refers to an image display device or a light emitting device that uses an organic light emitting element for its light emitting element. Also, the following modules are all included in the definition of the light emitting device: a module obtained by attaching to an organic light emitting element a connector such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of a TAB tape or a TCP; and a module in which an IC (integrated circuit) is directly mounted to an organic light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
Organic light emitting elements are elements that emit light upon application of electric field. It is said that organic light emitting elements emit light through the following mechanism: a voltage is applied between electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are re-combined in the organic compound layer to form excited molecules (hereinafter referred to as molecular excitons), and the molecular excitons return the base state while releasing energy to cause the organic light emitting element to emit light.
Molecular excitons generated in organic compounds take either singlet excitation or triplet excitation. This specification deals with elements that emit light from singlet excitation and elements that emit light from triplet excitation both.
In an organic light emitting element as above, its organic compound layer is usually a thin film having a thickness of less than 1 μm. In addition, the organic light emitting element does not need back light used in conventional liquid crystal displays because it is a self-luminous element and the organic compound layer itself emits light. The organic light emitting element is therefore useful in manufacturing a very thin and light-weight display device, which is a great advantage.
When the organic compound layer is 100 to 200 nm in thickness, for example, recombination takes place within several tens nanoseconds since carriers are injected based on the mobility of carriers in the organic compound layer and, counting the process from carrier recombination to light emission in, the organic light emitting element is readied for light emission in microseconds. Accordingly, fast response is also one of the features of the organic light emitting element.
Since the organic light emitting element is of carrier injection type, it can be driven with direct-current voltage and hardly emits noises. Regarding driving voltage, a report says that a sufficient luminance of 100 cd/m2 is obtained at 5.5 V by using a very thin film with a uniform thickness of about 100 nm for the organic compound layer, choosing an electrode material capable of lowering a carrier injection barrier against the organic compound layer, and introducing the hetero structure (two-layered structure) (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, no. 12, 913-915 (1987)).
Organic light emitting elements are drawing attention as the next-generation flat panel display elements for their characteristics including being thin and light-weight, fast response, and direct current low voltage driving. Also, being self-luminous and having wide viewing angle give the organic light emitting elements better visibility. Therefore, the organic light emitting elements are considered as effective elements for display screens of portable equipment.
These EL display devices are divided by driving methods into passive type (simple matrix type) and active type (active matrix type) and both types have been developed actively. Recently, efforts for developing EL display devices are centered around active matrix EL display devices that have active elements such as thin film transistors (TFTs) in their pixel portions.
Organic compound materials used in EL elements are classified into two types, namely, low molecular weight organic compound materials (hereinafter referred to as low molecular weight materials) and high molecular weight organic compound materials (hereinafter referred to as high molecular weight materials). Usually, these organic compound materials individually form one layer of amorphous thin film in an EL element. In other words, an EL element has a single layered structure consisting of one thin film, or has a multi-layered structure having plural thin films of different materials. In the case of the multi-layered structure, there is an interface wherever two organic thin films meet (hereinafter referred to as organic interface). The term thin film includes a thin film consisting of a single material and a thin film composed of plural materials. In the latter, a material serving as a host is doped (molecular dispersion) with a single or plural materials and this host-guest system is important. Each layer is several nm to several hundreds nm in thickness.
There is a carrier (holes and electrons) injection barrier in each organic interface between two organic thin films that are in contact with each other. The barrier for electrons is the energy difference in LUMO (lowest unoccupied molecular orbital) whereas the barrier for holes is the energy difference in HOMO (highest occupied molecular orbital). In addition, different kinds of organic thin films have different thermal and electric properties to reduce adhesion between the films. This may cause one of the films to peel off the other at the organic interface when the EL element is driven. From the reasons above, a desirable EL element is structured so as to have less organic interfaces.
When a low molecular weight material is formed into a film, vacuum evaporation is employed in most cases. An advantage of vacuum evaporation is that the conventional shadow mask technique can be used to pattern the formed film. Vacuum evaporation is also advantageous in that the purity of the material can be kept because it is dry process in vacuum.
However, vacuum evaporation also has problems and the biggest one regarding materials is that it cannot form a material into a film unless the material is volatile (or sublimable). In other words, vacuum evaporation cannot employ a material that is decomposed instead of being evaporated when heated no matter what the material has excellent light emission characteristics and carrier transporting characteristics.
When forming an organic compound layer from a low molecular weight material by vacuum evaporation, co-evaporation is often used to dope the material with a minute amount of pigment. It is technically difficult to achieve co-evaporation of a host material and a plurality of dopants simultaneously at a constant rate, and this is another drawback of forming a low molecular weight organic compound layer by evaporation.
On the other hand high molecular weight materials can be formed into films by several methods. For example, spin coating can be employed if the high molecular weight material is soluble and used in the form of solution. Although it has a drawback of wasting about 95% of the material prepared, spin coating is easy to obtain a large area light emitting element if the element emits light of one color using a single substance. Spin coating is also easy to dope the host with a plurality of dopants simultaneously if the solutions are adjusted in advance.
Under current circumstances, high molecular weight materials are dissolved in solvents for wet film-forming. This means that the high molecular weight materials have to be soluble in solvents. In short, a drawback of forming a high molecular weight organic compound layer by wet film-forming is that usually a material that is insoluble in a solvent cannot be used. Furthermore, it is difficult to obtain a thin film of high purity by wet film-forming.
Also, π conjugate polymers used in organic light emitting elements generally have small flexibility in π conjugate principal chain and have almost no solubility in solvents. It is therefore popular to introduce alkyl base, alkoxy base, or the like as a side chain in order to improve their solubility.
However, introduction of substituent brings about a problem of a change in color of light emitted from the compound. For instance, an organic compound material expressed as poly(1,4-phenylene vinylene) (hereinafter referred to as PPV) emits green light whereas an organic compound material which is obtained by introducing alkoxy base to PPV in order to improve the solubility and which is expressed as poly(2,5-dialkoxy-1,4-phenylene vinylene) (hereinafter referred to as RO-PPV) emits orange light.
Introduction of substituent (especially one that has strong electron absorption/donation power) may also lower the light emission efficiency of an organic light emitting element. This applies not only to high molecular weight materials but to organic compounds in general (Reference 2: Seiji Tokito, Hiromitsu Tanaka, Yasunori Taga, “Monthly Display Special Issue: Organic EL Display, Basics to the Latest News”, Techno Times Co., Ltd., pp. 98–99). Therefore, introduction of substituent as a measure to increase the solubility often leads to negative results in terms of light emission characteristics.
Furthermore, introduction of substituent means addition of a synthesizing step, which makes it difficult to manufacture the material at low cost.
As has been described, organic materials used in organic light emitting elements have to be volatile (sublimable) or soluble and, because of this restriction, an advantage of organic materials, namely, a wide choice of materials, is not fully utilized under the present circumstances. Also, in wet film-forming, improvement of solubility by introduction of a substituent cannot be achieved without lowering the light emission efficiency and increasing the manufacture cost.